3D effects are created by presenting different images to the left and right eyes of the viewer such that their visual system generates a sensation of depth. There are a number of different approaches to presenting an independent image to each eye, and many of them share the property that the optical separation of the “right” and “left” channels is not perfect.
This finite channel contrast is a result of various mechanisms, depending on the technique used to achieve separation, but the result is that in many cases, a faint “ghost” image of a bright region intended for the left eye can sometimes be seen by the right eye (and vice versa).
A conventional ghost compensation technique exists, but it only works if there is sufficient brightness available in the image to subtract a ghost image from. For example, if a white, distinct, object is displayed against a black background (such as a full moon in a dark night sky), then a displaced ghost image leaking from one eye's view would be seen against a black background in the other eye. Because the background is black, this ghost image cannot be adequately subtracted using the conventional ghost compensation technique, and so is visible.
The visibility of the artifact has two effects. First, the ghost images are distracting as they can be directly seen. Second, the ghost images are disruptive to the formation of the 3D sensation and so add to viewer fatigue.